Typically, I/O chips are made of lithium niobate (LiNbO.sub.3) or lithium tantalate (LiTO.sub.3). These I/O chips are used in optical systems including optical interferometers utilizing optical wave guides. The I/O chips can be either of a single type including transducers, filters, modulators, memory elements, and others or of several functional applications combined onto a single device.
Typically, suppliers of lithium niobate crystals furnish pieces that are commonly in the form of thin slabs. These thin slabs may be designated X-cut, Y-cut or Z-cut referring, respectively, to the X, Y or Z axes being normal to the broad face of the slab. Thus, an "X-cut, Y-propagation" describes a device having the X-axis normal to the broad face and the Y-axis in the direction in which light propagates within the wave guide.
Lithium niobate has found widespread application in laboratory and experimental systems. However, in order to make practical use of lithium niobate as an integrated optic device, numerous material problems still require solutions. One such problem is that lithium niobate devices still require a coupling device between themselves and optical fibers which will function over a wide range of environments (i.e., shock, vibration and temperature). This inability to develop an environmentally stable coupling device stems from lithium niobate's strong anisotropic thermal expansion properties and from movement between the waveguide and the optical fiber. A strong anisotropic thermal expansion property means that the dimensional changes in the material assooiated with a temperature change differs in different directions in the crystal. Lithium niobate exhibits a thermal expansion property along the Z-axis (the Z-axis being defined as the axis about which the crystal exhibits three-fold rotation symmetry) in the range between 2.times.10.sup.-6 per degree Centigrade to 7.5.times.10.sup.-6 per degree Centigrade (note: the variations being due to the various investigators' use of different materials, measurement techniques and over different temperature ranges), while the thermal expansion in the isotropic X or Y axes are in the range between 14.times.10.sup.-6 per degree Centigrade to 17.times.10.sup.-6 per degree Centigrade. Military applications require the integrated optic device to survive temperature fluctuations between -40.degree. C. to +80.degree. C. Movement between the optical fiber and the waveguide occurs primarily because of the environment in which the integrated optic device is used along with the anisotropic thermal expansion property of lithium niobate. In a gyroscope system, for example, the integrated optical device will have one degree of freedom which will sustain a substantial amount of the shock occurring along that axis while all three axes will undergo vibration. Since the light wavelength in many state-of-the-art applications is at the 850 nanometer range, the outside maximum allowable movement between the wave guide core with respect the optical fiber core or vice versa is on the order of tenths of microns. Any movement beyond this narrow tolerance usually causes unacceptable distortion to the output signal.
Optical fibers coupled to I/O devices made of lithium niobate are well known in the art. Generally these devices are large, complex and are sensitive to shock and vibration forces. An I/O chip bonded to a substrate and coupled to an optical fiber is disclosed in U.S. Pat. No. 4,750,800 and assigned to the same assignee as this application. The point of novelty of that patent lies in the selection of the substrate material and in the mounting orientation of the I/O chip to the substrate. The substrate material is chosen from materials having anisotropic thermal expansion characteristics similar to those of the I/O chip. That patent discloses the use of a fiber carrier in combination with the I/O chip and the fiber optic conductor. It is also known in the prior art to invert an integrated optical device (I/O chip) on its substrate so that the input region lies adjacent to the substrate.
U.S. Pat. No. 4,445,571 (Divens et al) discloses a metal coated tapered optical fiber which is coupled to a substrate and a method of fabrication. A metal layer is formed on the tapered portion of the cladding to prevent light from escaping from the optical fiber. The fiber and the I/O chip are joined together by means of an optical grade epoxy. The fiber is inserted into the I/O chip by means of a conical taper on the tip of the fiber and a corresponding conical cavity in the I/O chip itself.
U.S. Pat. No. 4,474,429 (Yoldas et al) shows the use of another tapered fiber and I/O chip where the junction is subjected to a temperature cycle which fuses glass constituents used to coat the junction of the optical fiber and the I/O chip.
In an article entitled "Fiber Connectors, Splicers and Couplers," by C. Kae, and G. Bickel of ITT Electro-Optical Products Division, Roanoke, Va., there is disclosed in the use of grooved substrate surfaces used to align the junction of two optical fibers.
In an article entitled "Fiber-to-Waveguide Coupling Using Ion-Milled Grooves in Lithium Niobate at 1.3-um Wavelength", by A. C. G. Nutt et al, published in Optics Letters, Vol. No. 9, Nov. 10, 1984, there is disclosed a method for connecting a fiber to a waveguide wherein a groove is cut in the waveguide and waveguide substrate. This article teaches that butt coupling the relative positions of the fiber and waveguide while paying attention to matching of fields is an unstable arrangement and is unsuitable for multiport devices. The article further teaches that there is a problem of alignment of silicone V grooves to the lithium niobate substrate because of the difference in coefficients of thermal expansion which may give rise to instabilities. Although a good approach, this attempt had several shortcomings. First, the ion milling of the grooves in the waveguide and the etching of the optical fibers is a slow, expensive and time-consuming process requiring great precision which allows for potential alignment problems between the waveguide and the optical fiber. Second, The etching of the fibers greatly reduces the polarization maintenance properties (i.e., stress zones) of the optical fibers which may exclude the use of birefringent optical fibers. Third, this approach was never actually demonstrated over a wide temperature range or for low loss connections.
Another attempt at providing a temperature-stable coupling device was the use of silicon V-grooves. Optical fibers were prepared by epoxying single-mode fibers into V-grooves etched onto a silicon chip. A silicon chip cover was then mated with the silicon chip containing the etched V-grooves so that the optical fiber cores were precisely and periodically spaced along a straight line. After this assembly of the silicon chip, the end faces of the optical fibers were polished, butt coupled to a corresponding lithium niobate waveguide, aligned and then attached using an optical adhesive. See E. J. Murphy et al, J. of Lightwave Tec., Vol. LT-3, No. 4, pp. 795-798 (August, 1985). Although this approach allows multiple optical fiber mountings to the waveguide, it also has problems, First, the silicon V-groove chip requires very great accuracy and precision in the etching of the V-grooves in order to allow for the correct alignment of the optical fiber cores to the waveguide core. Second, the silicon V-groove approach can only effectively use single mode optical fiber with a 1300 nanometer wavelength or the 850 nanometer wavelength with reduced performance. This approach has never been demonstrated using either polarization maintaining optical fibers or optical fibers at the 850 nanometer wavelength. Third, the silicon V-groove chip is relatively large in physical size. The silicon V-grooved chip and the integrated optical device have different thermal expansion properties. Hence, at the silicon-lithium niobate interface, a substantial thermal mismatch exists which can result in thermal instabilities which could destroy the bond between optical fiber and the waveguide. Finally, silicon V-grooves require excellent fiber core/clad concentricity. Disclosure Of The Invention
In a copending application, U.S. Ser. No. 07/103,325, a fiber carrier is disclosed as being made of a material which has a coefficient of thermal expansion in the plane of the carrier mounting surface which is substantially equal to the coefficient of expansion of that of the mounting surface of the I/O chip. In that application, it is also disclosed that the fiber carrier may be of the same material as the I/O chip. The I/O chip and the fiber optic carrier are attached to each other by the use of epoxy resin.
In this invention, we provide for a metallic bond between the I/O chip and a fiber optic carrier. By means of a metallic bond, a bond much stronger than that obtainable by use of epoxy resins and other adhesives is obtained.
We also provide for a metallic bond between the fiber optic carrier and the optical fiber.
It is an object of this invention to provide an effective coupling device between lithium niobate devices and optical fibers for operation over a temperature range between -40.degree. C. to +80.degree. C. or greater.
Another object of this invention is to provide an effective coupling device between lithium niobate devices and optical fibers which is more resistant to shock and vibration.
In this invention, the fiber carrier, when welded to the integrated optical device, provides an effective coupling between the optical fiber and the light port in a mounting surface of the I/O chip.
Ideally, both the fiber carrier and the integrated optical device should have a similar or identical anisotropic thermal expansion property along the same known optic axis. Both the fiber carrier and the integrated optical device isotropic thermal expansion properties (typically isotropic in any given plane perpendicular to the anisotropic axis) should also be substantially similar in magnitude to each other.
The fiber carrier is positioned with respect the I/O chip so that the carrier anisotropic axis is parallel to the I/O chip anisotropic axis and such that an auxiliary surface of the carrier is parallel to the device mounting surface. (The auxiliary surface need not have any particular or special relationship to the anisotropic axis; for example, if a plane, it need not necessarily be perpendicular to the anisotropic axis. See FIGS. 2A & 3 of U.S. Ser. No. 103,325.) The optical fiber may be bonded to the fiber carrier so that the plane of the optical fiber end surface lies in the plane of the carrier auxiliary surface. The carrier auxiliary surface is finally bonded to the I/O chip mounting surface so that the optical fiber end facing surface is placed in registration with the light port in the I/O chip. According to this invention, the fiber carrier may be constructed of a metal which has the same thermal expansion characteristics as a metal which is used for the substrate of the integrated optical device. In this configuration, it is also desirable to place the I/O chip upside down on its metallic substrate so that the light port will lie adjacent to the metallic substrate which is metallically bonded to the metallic fiber carrier.
In another embodiment of this invention, the fiber carrier may be constructed of the same lithium niobate material as the I/O chip. When this method is chosen, the fiber carrier will be coated with a metal by spraying, vacuum depositing, or any other known method of depositing metal on such a substance. Similarly, the I/O chip is metallized so that it can be subsequently welded to the metallized fiber optic carrier.
Still further, for either embodiment, the optical fiber itself may be metallized in order to provide for a welded cohesive connection between the optical fiber and the fiber optic carrier.
In this invention, the welding of the metallic surfaces may be accomplished by any known means of welding. Typical welding techniques may include a laser welder, a diffusion welder, soldering with another dissimilar metal and electrostatic bonding. In accordance with the present invention, the I/O chip is fabricated of LiNbO.sub.3 or LiTaO.sub.3 and the substrate material as well as the fiber carrier may also be made of lithium niobate or lithium tantalate if a metallized surface is used for creating the welded bond of the fiber carrier to the substrate.
In the art of interfacing I/O chips with optical fibers, the act of interfacing to an optical fiber is referred to as "pigtailing". Unless adequate and reliable pigtailing is achieved, integrated optical chips will have little utility outside of the laboratory environment. The disclosed technique of welding a fiber carrier to either the I/O chip, or to the substrate for the I/O chip, provides a strong, reliable connection. This connection will be able to survive substantial g forces encountered in harsh environments, as well as the broad temperature range from -40.degree. C. to +80.degree.0 C. and more. The welded junction is much stronger, less likely to creep, and is overall more reliable than a similar junction produced by use of adhesives such as epoxy.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, as illustrated in the accompanying drawing.